The present invention relates to dynamo electric machines, and more particularly to an alternating current generator which is capable of providing both three phase and single phase output power at different but relatively constant voltage levels.
In a conventional synchronous generator, control of the output voltage obtained from the stator windings of the generator can be obtained by energizing the field windings on the generator rotor with a D.C. current. The amplitude of the energizing current is proportional to the output voltage of the generator. One method of providing direct current for the energization of the rotating field winding is to obtain alternating current energy from the generator stator windings by means of a stationary transformer, rectify the output current of the transformer and apply the resulting D.C. signal to the rotating field winding through a slip-ring and brush connection.
A second method for energizing the field windings of the generator is through the use of a brushless excitation system having compounding and control field windings, such as that disclosed in U.S. Pat. No. 4,117,388. These field windings are energized with a rectified current proportional to the output voltage of the generator stator windings. The field windings induce a current in a rotating exciter winding. The current of the rotating exciter winding is rectified and applied to the rotating field windings of the generator as the energization signal.
In the past, a power current transformer was used to provide power in addition to the power obtained directly from the stator windings of the generator so that sufficient compounding energy will be provided to hold the generator output voltage relatively constant for all load variations from no load to anticipated full load. These transformers are specially designed to magnetically add the current and voltage excitation components to provide the necessary voltage regulating power.
Conventional synchronous generators having brushless excitation systems have been capable of producing either three phase or single phase output power at the same voltage level but not both types of power at different voltage levels. This is due to the fact that the loads on the generator may be considerably different under three phase conditions than under single phase conditions. Furthermore, the compounding energy may also have to be considerably different under the two conditions in order to maintain a relatively constant output voltage from the generator. Therefore, it was necessary to provide two different generators in those situations in which both single phase and three phase output power were desired. For example, a center pivot irrigation system utilized in farming requires 480 volt three phase power. To provide standby power in emergency conditions, a generator capable of providing 120/240 volt single phase output power is required. Therefore, a farm was required to be equipped with two separate generators to provide the necessary types of power, which involved a substantial cost.